Electrochemistry Dictionary - D

Daniell
cell: A very early non-rechargeable battery. It consisted of a glass
jar containing copper and zinc electrodes, each immersed in their respective
acidic sulfate solutions. The two solutions were separated by a porous clay
cylinder separator. It was a galvanic cell in which the spontaneous electrodissolution
of zinc and electroplating of copper provided the electrical current. It was
one of the earliest practical "laboratory" electrical sources; but, of course,
it was not much use outside the laboratory.

DC: Abbreviation of direct current.
However this term is also used in connection with dc voltage, that is, a steady
voltage that will cause a "dc current" to flow in a conductor, and also in connection
with dc power.

Decomposition potential (voltage): The
electrode potential (cell voltage) at which a "measurable" electrolysis current
begins to flow. This is a qualitative parameter since "measurable" is rather
subjective.

De-electronation: Alternative name of
an oxidation process.

Deferred-action battery: See reserve
battery.

Degreasing: Process for removal of grease,
oil, etc from metal surfaces in preparation for electroplating. Typically, the
metal is immersed in hot, strongly basic solution or in organic solvents to
remove and dissolve these coatings. See also electrolytic degreasing.

Deionization: See desalination.

Demineralization: See desalination.

Dendrite: A crystalline shape produced
by skeletal growth ("dendritic growth") resulting in "tree-like" appearance
(often with many branches) in metal deposition.

Depolarizer: An archaic expression (hardly
used any more) for a material added to a battery electrode for reducing the
polarization upon application of a current. It usually completely changed the
nature of the electrode reaction.

Deposition/dissolution: See metal deposition/dissolution
reactions.

Depth of discharge: For a rechargeable
battery: the fraction, usually expressed as a percentage, of the total electrical
energy stored in a battery by charging that was recovered by discharging at
a certain point of time. Contrast with state of charge.

Desalination: A process to produce clean
(potable) water from brackish or seawater. Electrodialysis is an electrochemical
technique often used for this purpose.

Desorption: The opposite process of
adsorption. The removal of the excess concentration of the adsorbate from the
vicinity of the solid surface.

Diffuse double layer: See the Gouy-Chapman
model of the double layer. Often simply called the "diffuse layer." (The diffuse
layer is not to be confused with the diffusion layer.)

Diffuse-junction potential: See liquid-junction
potential.

Diffuse layer: See diffuse double layer.

Diffusion: The movement of chemical
species (ions or molecules ) under the influence of concentration difference.
The species will move from the high concentration area to the low concentration
area till the concentration is uniform in the whole phase. Diffusion in solutions
is the most important phenomenon in electrochemistry, but diffusion will occur
also in gases and solids. The rate of diffusion (diffusional flux) is proportional
to the gradient of the concentration in the solution, with the proportionality
constant called the "diffusion coefficient."

Diffusion layer: A thin liquid boundary
layer at the surface of an electrode that is immobile. This is part of a rather
simplified and not strictly correct model (originally proposed by the early
electrochemist Nernst, and is often called the "nernstian hypothesis") that
works surprisingly well in most cases. The electrolyte solution is divided into
three distinct parts: the bulk solution and the two diffusion layers at the
surfaces of the electrodes. The bulk solution is assumed to be so well stirred
that the concentration of all species is uniform throughout. In this region,
mass transport occurs only through convection. While in the diffusion layers
mass transport occurs only through diffusion. Charge transport occurs through
electromigration everywhere. The thickness of the diffusion layer can vary typically
between the order of 0.01 centimeter in a stagnant solution and the order of
0.0001 centimeter in very well-stirred solution. (The diffusion layer is not
to be confused with the diffuse layer.)While the concept of the diffusion layer
is related to the concept of the hydrodynamic boundary layer, the two are not
identical and neither is their thickness. The structure of the diffusion layer
can be assumed to be relatively simple in the presence of a large excess of
supporting electrolyte, which is usually the case in electroanalytical applications
and in electrode kinetics research. Under these conditions, practically all
the electrical current is carried by the ions of the supporting electrolyte,
and the transport number of the reactant and the product is practically zero.
When the current is initially turned on, the ions of the supporting electrolyte
will migrate in the diffusion layer to/from the electrode (depending on their
charge). However, since they do not take part in any electrode reaction, their
concentration will increase/decrease at the electrode surface compared to that
of their concentrations in the bulk solution. This will start the diffusion
of these ions in a direction opposite to their migration. After steady-sate
is reached, the diffusion will completely cancel the migration, and the net
flux of these ions will be zero. They do not contribute to the mass transport
in the diffusion layer; however, they are responsible for all the charge transport,
that is, the resistance of the diffusion layer depends on the conductivity of
the supporting electrolyte. On the other hand, the reactant and the product
will diffuse to/from the electrode surface, and they will carry all the mass.
The situation remains the same even if either the reactant or the product is
an electrically neutral molecule. It is usually assumed that the concentration
of all species changes linearly between the electrode surface and the edge of
the diffusion layer. An example is the diffusion layer at the cathode surface
during electroplating of copper from a solution containing a small amount of
copper chloride and a large concentration of sulfuric acid. All the current
is carried by the ions of the sulfuric acid (hydrogen cations and sulfate anions)
but the only possible electrode reaction is the reduction of the copper ions
since the reduction of hydrogen ions cannot occur at the prevailing electrode
potential (copper is lower in the electromotive series than hydrogen). The situation
is much more complex in the absence of supporting electrolyte. Then both the
electromigrational and the diffusional flux of the reactant and product must
be considered.

Dipole: A pair of equal and opposite
electrical charges separated by a small distance. A dipole will align itself,
if possible, in the presence of other electrical charges according to the attraction
of opposite and repulsion of like charges. Externally electrically neutral chemical
molecules can have a dipole inside. E.g., water is a triangular molecule with
the oxygen at one corner and the two hydrogens at the other two corners. The
internal charge distribution is such that the hydrogen side has a slight excess
of positive charge and the oxygen end is correspondingly negative. A dipole
is characterized by its "dipole moment," the product of the charge and the separation
distance (coulomb times centimeter).

Dipole moment: See dipole.

Direct current: See current. Abbreviated
as "dc."

Discharge curve: A plot
of cell or battery voltage as a function of time, or of discharge capacity,
under a defined discharge current or load.

Discharge rate: The current withdrawn
from a battery. This rate is commonly expressed as a multiple of the rated capacity.
See C-rate.

Discharging: The opposite process of
charging. In this process the battery or capacitor supplies electricity to a
load (e.g., motor, light bulb). The term discharging is also used to describe
the neutralization of an ion during an electrode reaction. E.g., a metal cation
is said to be "discharged" to an electrically neutral metal atom during electroplating.

Disinfection of water: See brine electrolysis.

Dissociation: The process that may occur
when a chemical compound is dissolved in a solvent (e.g., water). The molecules
of the compound will break up ("dissociate") into two or more ions resulting
in an ionically conducting electrolyte solution. E.g., the common table salt
(sodium chloride) will dissociate into a single charged sodium cation and a
single charged chloride anion.

Dissolved-oxygen electrode: See Clark
electrode.

Divided cell: An electrochemical cell
in which the electrolyte is divided into two or more compartments by separators.
Such separation may be necessary for two reasons. The solutions around the anode
and the cathode may be different and it may be desirable to keep them from intermixing.
Alternatively, it may be desirable to keep the products of the reactions at
the anode and the cathode separated.

Divider: See separator.

DME: Stands for dropping-mercury electrode.

Donnan equilibrium: See Donnan potential.

Donnan potential: The electrical potential
difference between two solutions separated by an ion-exchange membrane in the
absence of any current flowing through the membrane. The concept of the Donnan
potential is analogous to that of the equilibrium electrode potential. Consider
two table salt (sodium chloride) solutions of different concentrations separated
by a cation-exchange membrane. The concentration difference will set up a diffusional
force driving the sodium chloride from the higher concentration solution into
the lower concentration solution through the membrane. However, the ion-exchange
membrane will permit only the passage of the positively charged sodium cations.
Consequently, excess positive electrical charges will accumulate on the low
concentration solution side of the membrane, while excess negative electrical
charges will accumulate on the high concentration side because of the negatively
charged chloride anions that are left behind. This charge separation will induce
an electrical potential difference that will drive the electromigration of the
sodium ions in the direction opposite to that of the diffusion. The overall
result will be that the net movement of the sodium ions into the lower concentration
solution will slow and eventually stop when the two opposing forces are equal
end the two opposing fluxes are equal. In this so called "Donnan equilibrium,"
the diffusional flux of the sodium ions in one direction will be equal to the
electromigrational flux in the opposite direction, resulting in net zero mass
transport and zero charge transport. The electrical potential difference across
the membrane under these equilibrium conditions is the "Donnan potential."

Double-junction reference electrode:
A reference-electrode assembly (for example a silver/silver-chloride electrode)
that is encased into a secondary containment vessel (typically a glass tubing)
filled with an electrolyte not containing chloride ions (often a high concentration
potassium nitrate solution). This second "internal electrolyte" of the reference
electrode assembly and the external electrolyte into which the whole assembly
is immersed are in ionic contact through a second separator (e.g., a porous
ceramic plug). The purpose of this arrangement is the avoidance of chloride
ion contamination of the test solution (many electrode reactions are strongly
catalyzed by chloride ions) at the price of increased liquid-junction potential.

Double layer: See electrical double
layer.

Double-layer capacitance: The measure
of the ability of an electrical double layer to store electrical charge as a
capacitor.

Double-layer current (density): See
capacitive current (density).

Double-layer range (or window): The
electrode potential range where an electrode behaves as an ideal polarized electrode.
In this potential range, the electrode potential is not positive enough that
any species in the solution could be oxidized and the potential is not negative
enough that any species could be reduced. In practical situations, there is
almost always a small residual current flowing that is faradaic in nature.

Drain:Withdrawal of current from a cell or battery.

Dropping-mercury electrode: A working
electrode arrangement for electroanalytical techniques, such as polarography.
Mercury is flowing continuously through a capillary tubing forming a small droplet
(typical diameter about 0.1 cm) exposed to the solution. The old drop falls
off and a new drop forms typically every few (3-6) seconds. The advantage of
this self-renewing electrode is that the effect of impurities in the solution
is minimized. Typically a new drop will form before impurities have a chance
to adsorb on the surface of the old drop to such an extent as to influence the
charge-transfer reaction. Abbreviated as "dme."

Dry cell: An early name for the non-rechargeable
battery that is still used occasionally. The early non-rechargeable batteries
were laboratory devices (see, e.g. the Daniell cell). To produce a practical
device, the electrolyte solution was "immobilized" by some gelling agent, and
the whole cell was sealed to permit its use in any position. Hence the name:
"dry cell." See also Leclanche cell. A cell in which the electrolyte is immobilized,
being either in the form of a paste or gel or absorbed in a microporous separator
material.

Dry charged cell: A cell
which is in its fully charged state but without electrolyte.

Dry-charged battery: See reserve battery.

Duplex electrode:Type of electrode system used in flat Leclanché multicell batteries,
formed by zinc coated on one side with carbon. It acts as the cathode current
collector for one cell and as the anode for the adjacent cell. (see 'bipolar
electrode').

Dynamic equilibrium: See exchange current
density.

Dynamic hydrogen electrode: A pseudo-reference
electrode assembly, simulating a reversible hydrogen electrode with an approximately
20 to 40 mV more negative potential. While its potential is less defined, it
has the advantage of not requiring a hydrogen gas supply. It is typically a
glass tube containing two internal electrodes, at least one of which is a platinized
platinum electrode, immersed in the same electrolyte solution as is the electrolyte
in the working cell, with the two electrolytes in ionic contact through a separator.
A small, constant current is enforced between the two electrodes with the platinized
platinum being the cathode, carrying typically 1 mA/cm2 current density, resulting
in a small amount of hydrogen evolution. This cathode is then used as the reference
electrode.